Distinct origins of magnetic -field -induced resistivity irreversibility in two manganites with similar ground states : Pr$_{0.5}$Sr$_{0.41}$Ca$_{0.09}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$

Our investigation of the magnetotransport in two charge ordered manganites with similar magnetic ground states reveals that the origin of magnetoresistance can not be concluded from the isofield resistivity, $\rho $(T, constant H), measurements alone. Both Pr$_{0.5}$Sr$_{0.41}$Ca$_{0.09}$MnO$_{3}$ (PrSrCa) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (LaCa) show a ferromagnetic transition (T$_{C}$ = 260 K for PrSrCa, 230 K for LaCa) followed by an antiferromagnetic transition (T$_{N}$ = 170 K for PrSrCa, 140 K for LaCa). These compounds show qualitatively similar magnetotransport : Below the irreversibility temperature T$_{IR}$, field cooled (FC) resistivity is lower than zero field cooled (ZFC) and decreases continuously with T, whereas the ZFC $\rho $(T, H) resembles $\ $the behavior of $\rho $(T, H = 0 T). The value of $\rho $(ZFC)/$\rho $(FC) is $\approx $ 10$^{4}$ at 5 K and $\mu_{0}$H = 7 T in both compounds. However, isothermal magnetic measurements suggest distinct origins of magnetoresistance : Field cooling enhances ferromagnetic phase fraction in LaCa whereas it drives PrSrCa into a metastable state with high magnetization. The distinct origins of magnetotransport is also reflected in other magnetic history dependent properties.